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Victor 4900 Programmable Calculator

This exhibit is dedicated to the memory of Johnny Nelson.

Johnny and his wife, Betty, were the owners of Nelson Office Equipment of San Luis Obispo, California from the mid-1930's through the late 1970's. Nelson Office Equipment was a premier Victor calculating machine dealer.

This machine is from the latter part of the 1970's, making it generally outside the area of interest for the Old Calculator Museum, however, part of the interest in early electronic calculators is the uniqueness of design and engineering that went into the old machines. While the Victor 4900 is a later desktop calculator, its use of an early microprocessor chip, combined with the fact that it is among the last of the desktop programmable calculators makes its inclusion into the museum applicable. By 1976, when the Victor 4900 was introduced, the concept of the microcomputer had begun to creep into the consciousness of the the public. With the introduction of the Apple II in April of 1977, it became clear that the days of the desktop programmable calculator were limited.

The Victor 4900 is the "top of the line" of a series of destkop programmable calculators that were initially introduced by Victor Comptometer Corporation in the mid-1973 timeframe. The series initially consisted of four machines, with the low-end machine being the model 4500 (priced at $995), continuing on with the 4600($1295), 4700($1595), and the top-of-the line 4800 at $1995. These machines all shared an identical architecture and basic functionality. The 4500 was the base machine, and did not include the magnetic card reader, so any programs would have to be manually re-entered any time the calculator was powered off (the program step and memory register storage was volatile, meaning its content was lost when power was removed). The 4600 was identical to the 4500, but added the mag card reader. The 4700 and 4800 calculators improved on the 4500/4600 by adding additional memory registers and program step storage. All of the machines in the series provide the standard four math functions, along with one-key percentage and square root operations. The machines all share two accumulator-style memory registers, and provide capacity of fourteen digits, with up to thirteen digits behind the decimal. All of the calculators are printing-only machines, utilizing a 120 character-per-second impact dot-matrix printer for recording calculations. All of machines provide for a settable decimal point location, a round-off function, and sophisticated programming functionality including conditional and unconditional jumps and subroutine branches to absolute step addresses or symbolic tags.

One may note that the featured machine in this exhibit isn't mentioned in the models above. That is because it wasn't part of the initial introduction of the 4x00-series calculators. The Victor 4900 became available later, and included significant improvements in functionality, including addition of advanced math functions, significantly enhanced memory and program step capacity, and additional programming capabilities. While using the same basic architecture, the 4900 provided these increased capabilities through essentially a "do-over" of the firmware from the earlier machines to pack more code functionality into the same amount of ROM space. This work resulted in the Victor 4900 being introduced in January of 1976, for a price of $2,195. Victor really needed to make these improvements, as by this time, competitors had vastly out classed the capabilities of the earlier Victor 4x00-series calculators. The 4900 brought Victor to par with the capabilities offered by many of its competitors in the moderately-priced programmable desktop calculator marketplace.

Inside the Victor 4900

All of the machines in the 4x00-series made use of an early microprocessor chipset to implement their functionality. The use of a microprocessor, as opposed to specialized calculator chips, was pioneered by the Japanese company Nippon Calculating Machine Co. (NCM), which was an early player in the Japanese calculator marketplace. NCM marketed its machines under the brand-name Busicom, and introduced their ground-breaking microprocessor-based Busicom 141-PF in October of 1971. The 141-PF utilized Intel's 4-bit 4004 microprocessor chip, combined with various support chips (RAM, ROM and I/O interfacing) to create a business-oriented desktop printing calculator. Busicom had originally engaged with Intel to develop a specific complex calculator chipset for their calculator, but Intel engineers felt that this was unnecessarily complicated, and came up with the idea for the small general-purpose microprocessor to replace the complex chipset that Busicom had envisioned. Busicom balked initially, but became convinced of the flexibility that the microprocessor brought to calculator design, and went ahead to implement the 141-PF with Intel's microprocessor.

The Rockwell PPS-4 Microprocessor Chip

In order to compete with Intel in the explosive market created by the microprocessor, a number of American semiconductor manufacturers got into the act and created their own microprocessor families. Among them was Rockwell International. Rockwell, though generally not well-known in the early history of electronic calculators, actually did have a major place, in that the company developed and fabricated the early MOS LSI IC's for Sharp that became the groundbreaking Sharp QT-8D calculator. Rockwell introduced their first microprocessor, the PPS-4, in 1972. At that time, there were only three other microprocessors on the open market; the Intel 4004 and 8008, and the IMP-16 from National Semiconductor. The Intel 8008 and the IMP-16 weren't well-suited for calculator applications, creating a great opportunity for Rockwell International to introduce their own microprocessor designed explicitly for use in calculators and other types of devices that required a modest amount of computing power at a low-price. The PPS-4 ended up in a number of different types of consumer products, including pinball machines, microwave ovens, point-of-sale terminals, and even some early handheld games. The PPS-4 was a good choice for the brains of a complex programmable desktop calculator, and since Victor had already forged a customer relationship with Rockwell for large scale integration (LSI) calculator chips in earlier calculators (such as the Victor 1800-series machines), it was a natural fit for Victor to adopt Rockwell's new microprocessor chips for use in their line of advanced programmable calculators.

The Three-Board "Stack" used in the Victor 4500 through early 4900 Calculators

It appears that two different versions of the 4900 were made, with the later version leveraging higher levels of integrated circuit technology, mainly higher-density ROM and RAM chips. It is possible, although not substantiated at this point, that the design improvements for the second generation 4900 were also incorporated into the earlier 4x00-series calculators to reduce their complexity and cost.

The first version of the 4900 was based on the same general design of the earlier 4x00-series calculators. The main electronics of the 4500 through 4800 and early 4900 calculators consisted of a three board stack, situated underneath the printer assembly. The boards in the stack are connected to each other by means of gold-plated square pins and sockets that plug the boards together. A system of four nylon spacers keep the boards properly spaced, and also provide a mounting method to secure the stack of boards in place. The top board in the stack (as visible in the photo above) contains the transistorized drivers for the seven printhead solenoids and various other actuators (mag card reader, printhead movement, and ribbon color change) used in the machine.

The First-Generation CPU Board

The next board in the stack is the main CPU board, containing the PPS-4 microprocessor (Rockwell part number 10660), peripheral interface chips (printer, keyboard, and mag card reader), system ROM (containing the PPS-4 code that runs calculator), and clock generation circuitry. The chips on this board have designations on the circuit board that indicate their function. Five chips are the ROMs that contain the PPS-4 code that runs the machine. The CPU is obviously labeled. A chip designated "CL" appears to be clock generation and support circuitry for the CPU. The "PR" chip likely provides control functions for the printer, and the "1 CHAR" chip likely is a ROM that contains the dot patterns for the characters and digits that the printer can print. The "LDR" chip, which is socketed (the rest of the chips are soldered directly to the board), is a mystery. It may have something to do with control for the mag card reader (which would simply be not installed on the Model 4500 due to it not having a card reader). This is just supposition, though.

The First-Generation RAM & Model-Specific-ROM Board

The bottom board in the stack consists of the RAM (random access memory) used for storing the working registers and programming steps (as well as state information), along with a single ROM that provides additional code for the operation of the machine.

The First-Generation Mag Card Read/Write Amplifier Board

A board located in the base of the calculator underneath the keyboard assembly provides the read/write amplifiers for the magnetic card unit. This is a surprisingly complex board that uses both discrete and integrated circuit technology. There are four channels of amplifiers, all of which are the same, indicating that the read/write head in the mag card reader is a four-channel head, allowing four bits of data to be read or written at a time.

The Mag Card Reader/Writer Assembly

The magnetic card reader mounts at the front of the printing assembly, and consists of a small motor, a rubber roller that transmits the motor rotation to a shaft with another rubber roller that moves the card through the mechanism, guides to keep the card aligned, and the read/write head itself. The card is inserted through a slot at the upper right corner of the calculator. When an action occurs (either manually or through program control) that requires magnetic card access, a solenoid activates to pull the card against the spinning rubber roller, and the motor activates, drawing the card into the machine, over the read/write head. Once the card has been read/written, the motor reverses, and pushes the card back out of the machine. Then, the solenoid releases, allowing the card to be removed by the user.

The Main CPU Board in later version Victor 4900 calculators

The later version of the 4900 combined the content of two of the boards in the stack (CPU and RAM/ROM boards) into a single board that is situated underneath the keyboard. Another board located under the printer assembly contains the read/write amplifiers for the mag card unit, as well as all of the driver circuitry. This arrangement eliminated the stack of circuit boards, simplifying interconnections between the boards, and reducing the need for expensive gold-plated board interconnections. The later-design CPU board has the PPS-4 microprocessor, all of the peripheral interface chips, and all of the ROM and RAM for the entire system. It was possible to put all of this on one board mainly because of improvements in the amount of data that could be stored on a single RAM or ROM chip, allowing fewer chips to provide the read-only and read-write memory needed for the system. The larter revision CPU board crams all of the code for the machine onto three ROM chips (versus six in the early version), and packs 4096 4-bit words of RAM onto four Mostek MK4027 4Kx1 RAM chips (versus eight earlier-technology chips on in the early version). It isn't known at this point when the later version of the Victor 4900 debuted, but it is clear from date codes in the exhibited machine (which is the later version) that the 4900 was still being produced as late as the early 1980's.

Early and Late Version Victor 4900 Front Panel

Along with the circuitry differences, there were other differences between the early and later model Victor 4900's. The front panel is one area that is visibly different between the two versions. The earlier version of the machine used incandescent lamp indicators behind colored plastic jewels to show the operational state of the machine. The Error indicator is red, the Memory 2 Selector indicator is green, the Memory 1 & 2 In-Use (in calculator mode) and Program Halt/Execute indicators(in programming mode) are yellow and blue respectively. The later version replaced the incandescent lamps (which are prone to burning out) with much more reliable discrete red LED (Light Emitting Diode) indicators. Along with these indicator changes, there were also changes in the calculators's mode selection switch, as well as printed nomenclature versus moulded-in nomenclature. Lastly, the later version of the 4900 changed the design of the keyboard assembly, though from the appearances, the keyboards are identical. The early version utilized a complex, custom keyswitch module that was designed to match Victor's requirements for a special keyboard "feel". This keyswitch design was patented by Victor as US Patent #3993884. The later design keyboard used off-the-shelf keyswitch modules produced by Cherry (sacrificing keyboard feel for reduced cost), that used sealed leaf-switch contacts, drastically simplifying the keyboard electronics. These changes to the keyboard and front panel, combined with the improvements in the electronics, significantly reduced complexity, and, as was most-important in the highly-competitive calculator market of the time, decreased manufacturing complexity and cost. This translated to the machines being able to be more competitively priced, as well as increasing the profit margin potential for each machine sold.

The 4900 shares much of the basic functionality of the earlier 4x00-series calculators. It has a capacity of 14 digits, with up to 13 digits behind the decimal. It provides two traditional memory registers, with 200 store/recall memory registers. The machine provides 1720 steps of program memory, with most functions consuming one step, though some more complex functions can take up to three steps. The machine is a printing-only calculator, using a dot-matrix impact printer to create printed output. The printer operates at 120 characters per second maximum, and prints unidirectionally, from right to left. The printer can print up to 32 characters per line. A magnetic card reader is provided that can record up to 128 program steps or 16 memory registers on each edge of a X" x Y" magnetic card, meaning that a single card can contain 256 program steps, or 32 memory registers.

The Printhead

The printing unit in the Victor 4x00-series calculators is quite conventional. The printhead consists of seven powerful solenoids, that, when triggered, drive a pin out the printing end of the printhead quickly and with significant force for a short period of time. The pin strikes the ribbon, leaving a dot on the paper. The pins are aligned in a vertical row, such that if all solenoids were activated at the same time, a vertical line of seven dots would be printed. The printhead is held at the right end of the carriage by spring tension. A motor with a clutch system connected to it allows the printhead to be pulled at a constant speed to the left as long as the clutch is activated. When the clutch is released, the printhead automatically returns to its home at the right end of the carriage. As the printhead moves to the left, a linkage system causes the ribbon to advance, assuring fresh ink for printing. Timing for the printing is generated by a metal strip with tiny slits cut into it. A photodiode and LED mounted to the printhead, and the light beam is interrupted by the slits as the printhead moves, creating timing signals that tell the printing control chip where the printhead is located, allowing the dots to be printed in the correct locations. A ROM chip provides the dot patterns for the various characters and numbers that the printer can print. A small solenoid can shift the ribbon upwards, allowing red characters to print, as the calculator uses a bi-color ribbon, with black ink at the top half of the ribbon, and red ink at the bottom half.

Victor 4900 Keyboard Layout

The machine provides the standard four math functions along with single-key percentage and square root functions. A programmed function key ([PR F]) provides access to more advanced math functions such as trigonometric and logarithmic functions via a two-key sequence. Addition and subtraction operate adding-machine style, with the [+ I] and [- II] keys (more on the I and II designations later) adding or subtracting the last number entered to/from the accumulator register. The [S] key prints out the current content of the accumulator register, and the [T] key does the same thing as the [S] key, but clears the accumulator after the printout occurs. Multiplication and division use the [=] key to calculate the result. The machine has a four function constant, retaining a constant value for repeated calculation. For example, to generate the power of two, all that is necessary is for the user to enter [2] [X] [2], then repeatedly press the [=] key to generate each successive power of two. The [%] key is used in conjunction with the [X] and [÷] keys to calculate percentages. For example, to calculate 15% of 250, one would enter 250, press the [X] key, then enter 15, and press the [%] key, which prints the 15 with a % sign after it, and serves to complete the operation, printing the result of 37.50. The [√] key immediately completes any calculation in progress, and prints the result, followed by "SQ", then calculates the square root of the result, and prints it. An example would be adding 1+1, then calculating the square root of the result. This would be entered as [1] [+ I] [1] [√]. The printout would be "2 SQ" (the result of 1+1), followed by "1.4142123562373" (the square root of 2) printed on the next line.

The memory functionality of the machine is generally conventional, with the [M+] and [M-] keys adding/subtracting the content of the accumulator to/from the memory register. The [MR] key recalls the memory register into the accumulator, and the [MRC] key does the same thing, but clears the memory register after the recall occurs. The [=+] and [=-] keys serve to terminate a math operation and adds/subtracts the resulting answer to/from the memory register. The twist to the two memory registers is that in order to access the second memory register, the [M2] key is pressed before any memory function key to tell the machine to perform the following operation on memory register #2. When the [M2] key is pressed the "M2 SEL" indicator on the front panel lights up to tell the operator that the next memory operation will affect memory register #2. Once a memory operation is selected, the "M2 SEL" indicator goes off. When either of the memory registers is non-zero, indicators on the front panel (labeled "M1" or "M2") light to show that the memory register is in use.

The [DS] and [DR] keys are used to access the addressable memory registers. The addressable memory registers are independent of the two "built-in" accumulating memory registers. From the keyboard, these registers can only be used to store and recall a number. Through the programming function of the machine, there are a number of different methods to utilize the addressable memory registers, including indirect addressing and increment/decrement instructions. The 4900 has 200 of these addressable memory registers, designated 000 through 199. To store a number into one of the addressable memory registers, the [DS] key is pressed, then the address of the memory register is entered. At this point, the value in the accumulator is stored in the addressed memory register. For example, if the accumulator contains 15.54, and [DS] [0] [5] [2] were pressed, 15.54 is stored in memory register 52. The recall an addressable memory register to the accumulator, the [DR] key is pressed, followed by the three digit memory register number. It is always necessary to enter three digits after use of the [DS] and [DR] keys, thus requiring entry of the leading zero for memory registers 0 through 99. If an attempt is made to enter a memory register address that is out of range, the "ERROR" indicator on the front panel lights, and the calculator locks up until the [C] key is pressed to clear the error condition.

More keyboard keys provide additional general functions. The [EX] key exchanges the two operands of math functions (for example, 4 ÷ 1 becomes 1 ÷ 4 when the [EX] key is used). The [CS] key changes the sign of the number in the accumulator to make it negative. The [C ALL] key clears the working registers of the calculator, and both built-in memory registers, but does not clear the addressable memory registers. The [C] key clears overflow and error indications, and the working registers of the calculator, along with canceling any math operation that was in progress.

The 4x00-series machines provide full-floating decimal point, however, for various business and accounting uses, a fixed decimal position can be set, using the [. SET] key. When the decimal point position is fixed, results are printed using the set decimal point position, except in the case of functions that generate results with more digits behind the decimal point, such as square root and other higher- level math functions. When the [. SET] key is pressed, the machine prints out the current decimal point selection with a period in front, for example, if the current setting is 2 digits behind the decimal point, the print out would read ".2". Then, a single digit from zero through nine is entered to set the new decimal point position, and the printer then prints out the new setting in the same form, e.g., if a 6 was pressed after the [. SET] key, the printout would be ".6". An unusually labeled key, [∩], provides a round-off feature. When the [∩] key is pressed the current number in the accumulator is rounded to the number of digits selected by the current decimal point position setting, and the result printed. For example, if the accumulator contains 1.414213562, and the decimal point position is set at 6, pressing the [∩] key would result in "1.414214 RO" being printed, with the "RO" indicating that the round-off function created this result.

Mode Selection Slide Switch on Front Panel

On the front panel of the 4x00-series calculators is a slide switch that selects the operational mode of the calculator. For manual operation of the calculator, there are two modes. There are four modes of operation for programming-related functionality, and there are two modes for operation of the magnetic card reader/writer. For manual operation there are "ADD" and "CALC" modes. The "ADD" mode of the calculator is an unusual feature. In this mode, the calculator behaves as a basic adding machine. Only the add/subtract math functions, and the two built-in memory register functions ([M+], [M-], [M2], [MRC], and [MR]) of the calculator are operational in this mode. For basic business math, this mode is convenient, as it eliminates errors that could creep into calculators by disabling all of the other function keys. In "CALC" mode, the full calculation functions of the machine are available.

For programming functionality, the modes are "PRG", "LST", "RUN", and "TRC". In "PRG" mode, the calculator goes into program entry mode, where keypresses are stored in program memory as program steps, with the printer listing out each operation as it is entered. In "LST" mode, the calculator will print out a listing of stored program steps. In "RUN" mode, programs stored in the calculator can be executed, and finally, in "TRC" mode, programs are executed, but the process of running the program is printed out step-by-step on the printer as each program step is executed. This mode is helpful for debugging programs.



Early and Late Victor Mag Card Envelopes (Front & Rear)

The two modes for control of the magnetic card reader/writer are "WRT" and "RD", which stand for "Write" and "Read" respectively. When in "RD" mode, and a magnetic card is inserted in the reader, pressing the "+ I" key will cause the card to be read into either program or addressable memory storage. When in "WRT" mode, the content of program or addressable memory registers are written out to the magnetic card when the [+ I] is pressed.

Listing and Execution of Simple Program to Calculate Square Root of 1 through 10

The programming features of the Victor 4900 are quite extensive, including absolute and relative (by label) jumping and branching, with an extensive repertoire of decision-making commands. Subroutine branching and return instructions are also available. It is also possible to programmatically read and write magnetic cards, allowing mag cards to hold records of data. With the large amount of program step memory on the 4900, and the ability to read and write magnetic cards as part of a program, it is possible to build complex programs that approach the capabilities of small mini- and microcomputer systems of the day.

Sincere thanks to Marilyn Nelson for donation of two Victor 4900 calculators, some magnetic cards, and a dust cover which were owned by her father, Johnny Nelson.
Special thanks to Mr. Bill Ewing for donation of an original programming manual and some magnetic cards for the Victor 4900 calculator.
Sincere thanks to Mr. Alfredo Logioia for providing scans of the Victor 4900 Operator and Programming Manuals to the Old Calculator Museum.
Text and images Copyright ©1997-2017, Rick Bensene.